Fiber laser pumping of bismuth-doped fiber amplifiers.

Abstract

A bismuth-doped optical fiber amplifier (BDFA) system in which a bismuth-doped optical fiber (BDF) is pumped by a fiber laser pump (rather than a semiconductor pump). Higher power fiber laser pumps enable overpumping of the BDF, providing benefits to fiber laser-pumped BDFAs that are inherently not possible with lower power semiconductor pumps. This disclosure relates generally to optical fibers, and more particularly to optical fiber cables.

Description

[Technical Field]

[0001] [Reference to Related Applications] This application, entitled "Fiber Laser Pumping of Bismuth-Doped O-Band Amplifier," by DiGiovanni et al., which is incorporated herein by reference in its entirety, claims the benefit of U.S. Provisional Patent Application No. 63 / 394,474, filed August 2022.

[0002] FIELD OF THE DISCLOSURE This disclosure relates generally to optics, and more particularly to optical amplifiers. [Background technology]

[0003] Bismuth-doped fiber (BDF) for BDF amplifiers (BDFA) has been used in the wavelength range of 1265 nm to 1345 nm. Traditionally, within this wavelength range, BDFs are pumped using quantum dot (QD) semiconductor lasers coupled to single-mode fiber (SMF), as shown, for example, in U.S. patent application Ser. No. 17 / 724,362, filed Mar. 8, 2021, and PCT application PCT / US19 / 51024, filed Sep. 13, 2019. Summary of the Invention

[0004] The present disclosure teaches a bismuth-doped optical fiber amplifier (BDFA) system in which a bismuth-doped optical fiber (BDF) is pumped by a fiber laser pump (rather than by a semiconductor pump). Higher power fiber laser pumps enable overpumping of the BDF, which inherently provides benefits to fiber laser-pumped BDFAs that cannot be achieved with lower power semiconductor pumps.

[0005] Other systems, devices, methods, features, and advantages will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of this disclosure, and be protected by the accompanying claims.

[0006] Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals indicate corresponding parts throughout the several views. [Brief explanation of the drawings]

[0007] [Figure 1A] FIG. 1 is a block diagram illustrating one embodiment of a forward-pumped (or co-directionally pumped) bismuth-doped optical fiber amplifier (BDFA) for an exemplary test configuration. [Figure 1B] FIG. 1 is a block diagram illustrating one embodiment of a forward-pumped (or co-pumped) BDFA in an exemplary telecom environment. [Figure 2] FIG. 1 is a block diagram illustrating one embodiment of a backward-pumped (or counter-pumped) BDFA. [Figure 3] 1 is a graph showing optical pump power (W) plotted against power (W) for one embodiment of a BDFA, with the graph inset showing the temperature dependent wavelength shift of a cladding-pumped fiber laser pump. [Figure 4] 1B is a graph showing gain and noise figure (NF) plotted against wavelength for the embodiment of the forward-pumped BDFA shown in FIG. 1A, with the graph inset showing the BDFA output spectrum with respect to wavelength for the embodiment of the BDFA of FIG. 1A. [Figure 5] 1B is a graph showing bit error rate (BER) plotted against channel power for the embodiment of the forward-pumped BDFA of FIG. 1A. [Figure 6]3 is a graph showing gain and NF plotted against wavelength for the embodiment of the backward-pumped BDFA of FIG. 2. DETAILED DESCRIPTION OF THE INVENTION

[0008] Traditionally, bismuth-doped fiber (BDF) for BDF amplifiers (BDFAs) in the 1265 nm to 1345 nm wavelength range is pumped using a semiconductor quantum dot (QD) laser coupled to a single-mode fiber (SMF). To properly pump a BDF so that the signal operating wavelength range is approximately 1265 nm to approximately 1345 nm, the SMF-QD semiconductor pump operates in the pump wavelength range of approximately 1190 nm to approximately 1200 nm. Furthermore, approximately 20% of the signal bandwidth overlaps with the OH absorption peak, reducing the BDFA gain and increasing the noise figure (NF). Because the pump wavelength (1190 nm to 1200 nm) lies between indium gallium arsenide (InGaAs) and indium phosphide (InP) semiconductor technologies, pumping can currently only be achieved with an SMF-QD semiconductor laser pump. As a result, BDFA pumps are less power efficient, more expensive, and have limited availability. In other words, the use of SMF-QD semiconductor pumps results in lower power conversion efficiencies in this operating wavelength range due to the inherently lower power associated with semiconductor pumps.

[0009] To address the drawbacks associated with SMF-QD semiconductor pumps, this disclosure teaches a BDFA system in which the BDF is pumped by a fiber laser pump (rather than by a semiconductor pump). Higher-power fiber laser pumps enable overpumping of the BDF, inherently providing benefits to fiber-laser-pumped BDFAs that cannot be realized with lower-power semiconductor pumps. One preferred embodiment demonstrates a BDFA with a BDF pumped by a conversion stage containing a single commercial-off-the-shelf (COTS) low-brightness uncooled multimode (MM) laser diode via a double-clad ytterbium (Yb)-doped fiber (YDF). The preferred embodiment has a gain greater than 20 dB (specifically, a gain of approximately 29.3 dB) in the wavelength range from approximately 1255 nm to approximately 1355 nm (a bandwidth of approximately 17.6 terahertz (THz)). For preferred embodiments, the NF is less than about 5.2 dB (specifically, about 4.6 dB at a center wavelength (λ) of about 1300 nm and an input power (Pin) of about −20 decibel milliwatts (−20 dBm)), and the BDFA has an electrical consumption of about 8.1 watts (about 8.1 W) to about 9.6 W or about 9.8 W over a temperature range of about 20 degrees Celsius (about 20°C) to about 70°C.

[0010] Having provided a broad technical solution to the technical problem, as well as one preferred embodiment of the technical solution, reference will now be made in detail to the description of the embodiments illustrated in the drawings. While several embodiments will be described in conjunction with these drawings, there is no intent to limit the disclosure to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents.

[0011] 1A is a block diagram illustrating one embodiment of a forward-pumped (or co-pumped) BDFA system 100. As shown in FIG. 1A, the embodiment of BDFA system 100 includes an amplifier stage 102, shown for illustrative purposes as an original band (O-band) amplifier stage operating in the wavelength range of about 1260 nm to about 1360 nm. Those skilled in the art will understand that although the O-band is shown as an example, BDFA system 100 and amplifier stage 102 can also operate in the extended band (E-band, ranging from about 1360 nm to about 1460 nm), the short wavelength band (S-band, ranging from about 1460 nm to about 1530 nm), or any other operating wavelength band in which a BDFA can be configured to operate.

[0012] The amplifier stage 102 then includes a bismuth-doped fiber (BDF) 104. For illustrative purposes and to illustrate experimental results, the BDF 104 is shown as a two-hundred-twenty-meter (220 m) BDF with a bismuth (Bi)-doped phosphorus silicate glass BDF core prepared by modified chemical vapor deposition (MCVD). The BDF core diameter is approximately eight micrometers (approximately 8 μm), and the BDF cladding diameter is approximately 125 μm. An embodiment of the BDF 104 has a numerical aperture (NA) of approximately 0.13 (approximately 0.13) and a cutoff wavelength (λ) of approximately 1180 nm, which allows for single-mode (SM) operation in the O-band. The measured fiber loss at a pump wavelength of approximately 1150 nm was approximately 0.3 dB / m, as measured by the cutback method. Those skilled in the art will appreciate that the length, core diameter, cladding diameter, and other characteristics of the BDF 104 can be varied to accommodate various system requirements.

[0013] In turn, the amplification stage 102 further comprises an output fiber Bragg grating (FBG) 106 optically coupled (e.g., by fusion splicing or other known optical coupling process) to the BDF 104 at the BDF output, and a wavelength division multiplexer (WDM) 108 having an output (or WDM output) optically coupled to the BDF 104 at the BDF input. One embodiment of the output FBG 106 has a reflectivity of approximately 99% to prevent pump leakage, improve amplifier gain by approximately 1 dB, and improve noise figure (NF) by approximately 0.3 dB. An exemplary embodiment of the WDM 108 is a thin-film filter-based WDM known in the art. In the embodiment of FIG. 1A, the input to the WDM 108 (or WDM input) is the pump input 110 to the amplification stage 102.

[0014] Amplification stage 102 further comprises an input optical isolator 112 that is also optically coupled to the WDM input, and an output optical isolator 116 that is optically coupled to the output FBG 106. For the embodiment of Figure 1A, the input to input optical isolator 112 is also amplifier stage signal input 114, and the output of output optical isolator 116 is amplifier stage output 118.

[0015] In addition to the amplifier stage 102, the BDFA system 100 includes a conversion stage 120 that pumps the amplifier stage 102. The conversion stage 120 comprises a pump laser diode (LD) 122, which for illustrative purposes is shown as a 10 watt (10 W) rated multimode (MM) uncooled pump LD for providing pump light with an operating center λ of about 915 nm. Particular embodiments of the pump LD 122 have an optical power vs. current input slope (LI slope) efficiency of about 0.88 to about 0.82 and operate at a threshold current of about 510 milliamps (about 510 mA) to about 630 mA at temperatures ranging from about 20°C to about 70°C.

[0016] The conversion stage 120 further includes a high-reflection (HR) FBG 124 optically coupled to the pump LD 122. In some embodiments, the HR FBG 124 has a reflectivity of approximately 99%. The optically coupled HR FBG 124 is a gain-doped FBG 126, which is shown in FIG. 1A as a 30 m double-clad ytterbium-doped optical fiber (YDF) with an approximately 6 μm core diameter and an approximately 125 μm cladding diameter. The output of the gain-doped fiber 126 is optically coupled to an optical coupler (OC) FBG 128. In the conversion stage 120 of FIG. 1A, the OC FBG 128 has a reflectivity of approximately 75%. In some embodiments, the HR FBG 124 is written with approximately 99.9% reflectivity and a 3 dB bandwidth of 1 nm, and the OC FBG 128 is written with approximately 75% reflectivity and a 3 dB bandwidth of 1 nm, thereby enabling lasing of the gain-doped fiber 126. To the extent that the OC FBG 128 is present at the conversion stage output 130, the OC FBG 128 is optically coupled to the input of the WDM 108 located at the amplification stage input 110.

[0017] The output optical power (W) of conversion stage 120 at a λ of about 1150 nm versus power (W) at various temperatures (e.g., about 20.0°C, about 45.0°C, and about 70.0°C) is shown in graph 300 of FIG. 3. Graph 300 shows an overall electrical-to-optical power conversion efficiency ranging from about 0.25 to about 0.20 for two watts (2 W) pump power at a λ of about 1150 nm. Note that conversion stage 120 operates relatively kink-free up to about 2.25 W of output power, although kinks can be caused by pump laser power jumps. Therefore, to generate 2 W of pump power at 1150 nm, the current and voltage of the pump LD 122 are about 4.6 amperes (A) and about 1.77 volts (V), respectively, at about 20° C., while the current and voltage of the pump LD 122 are about 5.3 A and about 1.84 V, respectively, at about 70° C. Inset 350 of FIG. 3 shows the spectra of the conversion stage 120 at 2 W of output power for both about 20° C. and about 70° C. Specifically, the spectrum of inset 350 shows an amplified spontaneous emission (ASE) level of about 57 dB and a temperature-induced wavelength shift of about 0.5 nm from about 20° C. to about 70° C.

[0018] Finally, the amplification stage 102 is pumped by a fiber laser-based conversion stage 120, thereby enabling overpumping of the BDF 104, which is not practically feasible with conventional SMF-QD semiconductor pumps. Furthermore, the power consumption of the disclosed BDFA system 100 is similar to that of a conventionally pumped BDFA operating over the same portion of the O-band at approximately 20°C. At approximately 45°C, the power consumption of the disclosed BDFA system 100 is 1.7 times lower than that of a conventionally pumped BDFA. At approximately 70°C, the power consumption of the disclosed BDFA system 100 is 3.5 times lower than that of a conventionally pumped BDFA.

[0019] To further clarify what is meant by overpumping in this disclosure, some embodiments explicitly define overpumping as when the BDF 104 is pumped with more pump power than is practical with the semiconductor pump. Those skilled in the art will understand that this threshold is not numerically recited for this particular embodiment because, once exceeded, it is impractical to add more pump power with the semiconductor pump. For example, if approximately 500 mW is available from a single semiconductor laser operating at a particular wavelength (λ1) to generate a particular gain (G), overpumping is the pump power above the pump wavelength (λ2) required to exceed that gain (G). Note that λ1 may be the same as or different from λ2. In other words, if a length of BDF generates approximately 20 dB of gain when pumped with approximately 500 mW of semiconductor pump power at approximately 1195 nm, anything above that level at the same pump wavelength (approximately 1195 nm) is overpumping (or excessive pumping). However, if about 750 mW of pump power at about 1150 nm is required to produce the same gain of about 20 dB, then anything above about 750 mW is overpumping (or excessive pumping). It should be noted that even for the same BDF (meaning a BDF with the same Bi concentration, process parameters, etc.), the optimal fiber length may differ for different pump wavelengths (λ2). For example, a BDF about 170 meters (about 170 m) may require about 500 mW of pump power at about 1195 nm to produce about 20 dB of gain (meaning the overpumping threshold is about 500 mW for that particular fiber), while a BDF about 145 m may require about 750 mW of pump power at about 1150 nm to produce the same gain of about 20 dB (meaning the overpumping threshold is about 750 mW).

[0020] For other embodiments, overpumping is explicitly defined as providing enough power to automatically distribute pump power along all sections of the BDF, with some pump power lost, for example, when it is impractical to provide optimal pump power to all sections of the BDF (e.g., requiring too many pump stages along the gain fiber (BDF)). In this embodiment, multi-mode (MM) pump diodes capable of generating several watts of power are used, thereby allowing pump powers of several watts, or even 10 W or more, or even 100 W or more.

[0021] In yet another embodiment, overpumping is explicitly defined as the ratio of the desired threshold pump power to the launch pump power (i.e., threshold / launch) so that every section of the gain fiber (e.g., every section of the BDF) is pumped above the desired threshold. For example, if the desired threshold power is about 150 mW and a launch pump power of about 1 W is required to pump every section of the BDF above about 150 mW, the overpumping level is about 15% (i.e., 150 mW / 1 W = 15%). In another example, if a launch pump power of about 2 W is required for every section of the BDF to be pumped above the threshold power of about 150 mW, the overpumping level is calculated to be about 150 mW / about 2 W = about 7.5%. Other calculations include examples where a starting power of about 2 W relative to a threshold power of about 0.2 W requires an over-pumping level of about 10% (i.e., about 0.2 W / about 2.0 W=about 10%), a starting power of about 1.0 W relative to a threshold power of about 0.15 W requires about 15% over-pumping, etc. In embodiments that explicitly define over-pumping in terms of threshold power to starting power ratio, a BDF pumped at a threshold to starting ratio of about 10% is considered over-pumped.

[0022] Ultimately, overpumping under each of these explicit definitions is a pump power level greater than can be practically implemented using semiconductor pumps, as will be understood by those skilled in the art.

[0023] BDFA system 100 further comprises a signal transmitter 134 optically coupled to amplifier stage input 114 through a transmission fiber 138 and a variable optical attenuator (VOA) 140. In some embodiments, transmission fiber 138 is single-mode fiber (SMF), and in the embodiment of FIG. 1A , transmission fiber 138 is shown as a 20 km to 53 km length of fiber conforming to the International Telecommunication Union Telecommunication Standardization Sector (ITU-T G.652) Recommended G.652 Characteristics for Single-Mode Optical Fiber and Cable, which is an industry standard familiar to those skilled in the art.

[0024] The signal receiver 136 is optically coupled to the amplifier stage output 118 via a bandpass filter (BPF) 144 and another VOA 142. For some embodiments demonstrating the suitability of the BDFA system 100 for near real-time network data transmission using four hundred gigabits per second (400 Gb / s) over a nominal distance of ten kilometers (10 km), an optical network tester (ONT) 132 was used that includes both the signal transmitter 134 and the signal receiver 136. The ONT 132 is capable of transmitting 16×25 Gb / s, 2 31 A -1 pseudorandom binary sequence (PRBS) data lane was generated. Internally to the ONT 132, the lane was converted to a 4 x 50 Gbaud / s pulse amplitude modulated (PAM) signal and encoded onto four coarse wavelength division multiplexing (CWDM) channels at approximately 1272 nm, 1292 nm, 1310 nm, and 1330 nm using an external modulator. The baud rate was selected for its high sensitivity to noise. The signal receiver 136 in this embodiment is a quad small form factor pluggable (SMFP) double density (DD) receiver, and the wavelength division multiplexing (WDM) signal is separated by a 38 nm 3 dB bandwidth optical filter and converted back to an electrical on-off keying (OOK) signal.

[0025] It should be appreciated that in other embodiments, the signal transmitter 134 comprises a tunable laser source (TLS) optically coupled to the amplifier stage signal input 114, and the signal receiver 136 comprises an optical spectrum analyzer (OSA) optically coupled to the amplifier stage output 118. It should be appreciated that embodiments using a TLS and an OSA as the signal transmitter 134 and the signal receiver 136 facilitate measurements of gain and NF. As an example, plot 400 of FIG. 4 shows the measured gain and NF for the particular configuration shown in FIG. 1A when the input power at the BDFA is set to −20 dBm and −10 dBm. Inset 450 shows a set of BDFA output spectra for an input power of −20 dBm (with a resolution bandwidth (RBW) of 0.1 nm). Inset 450 shows a gain peak of 29.3 dB at approximately 1300 nm, with a corresponding NF of approximately 4.6 dB. Across the entire λ range, the NF was less than approximately 5.2 dB, except at approximately 1255 nm, where the measured NF dropped to 6 dB due to a sudden increase in WDM loss. As shown in Figure 4, at -20 dB input signal power, the BDFA exhibits >20 dB gain between approximately 1255 nm and approximately 1355 nm for λ (a 17.6 THz bandwidth). At -10 dB input signal power, the gain was 2.7 dB lower (compared to the gain for a -20 dBm input signal) and the NF was 0.2 dB higher (also compared to a -20 dBm input signal). The measured data also showed a temperature dependence, with the gain peak shifting to approximately 1305 nm at approximately 70 °C (compared to approximately 20 °C), the gain decreasing by up to 1.1 dB, and the NF increasing by up to 0.3 dB across the wavelength range.

[0026] Data on the conversion stage noise sensitivity was also collected for the forward-pumped BDFA system 100. The data, in the form of waterfall curves, are shown in graph 500 of FIG. 5. The forward-pumped BDFA system 100 may be more susceptible to noise than the backward-pumped BDFA system 200. This is because the pump and signal propagate in the same direction in the forward-pumped BDFA system 100. The effects of amplified spontaneous emission (ASE) are minimized by placing an external BPF 144 with a 3 dB bandwidth of 1 nm in the signal transmission path before the signal receiver 136 (specifically, the QSFP-DD receiver). All four channels were amplified, but only the bit error rate (BER) of the approximately 1310 nm wavelength channel was measured because approximately 1310 nm was the desired signal wavelength for O-band communications. The total power was approximately 6 dB higher than the single-channel power. The input signal power (at the input to the BDFA) was set to about 2.5 dBm when λ was about 1310 nm to further reduce the ASE. As shown in Figure 5, comparing the amplified transmission (i.e., 2.5 dBm, -10 dBm, -16 dBm, and Pin of -16 dBm after 53 km of SMF) with the unamplified back-to-back transmission, the ASE was reduced by 5 × 10 -10 The small difference between the Δt and Δt values indicates that there is no significant conversion stage noise associated with the forward pumping scheme. The smooth waterfall curve also indicates negligible conversion stage noise.

[0027] Replacing the BPF144 with 20 km of G.652 fiber resulted in an average BER of 4.5 × 10 for all four CWDM channels over 8 hours of operation with a receiver signal power of 6 dBm and a total input power to the BDFA of 0 dBm. -6 It was.

[0028] Experimental results demonstrated several embodiments of the BDFA system 100 operating over a standardized portion of the O-band with >20 dB gain over a λ range from approximately 1255 to approximately 1355 (with input signal power of -20 dBm) and a typical noise figure (NF) below 5.2 dB, pumped by a single COTS high-brightness uncooled MM laser diode via a YDF-based conversion stage. The BDFA gain and NF were evaluated using an amplified spontaneous emission (ASE) spectral interpolation method known in the art. Specific examples demonstrate that embodiments of the BDFA system 100 operate over a temperature range of approximately 20°C to approximately 70°C with overall power consumption similar to or better than prior art single-mode (SM) diode-pumped amplifiers operating over the same portion of the O-band. Embodiments of the BDFA system 100 are suitable for data transmission by amplifying a 50 GBaud / s PAM-4 signal from a pluggable module over an approximately 17.6 THz bandwidth range. In the embodiment shown in FIG. 1A, the BDFA had a polarization dependent loss (PDL) of less than 0.15 dB and a differential group delay (DGD) of less than 0.3 picoseconds (ps).

[0029] 1B is a block diagram illustrating one embodiment of a forward-pumped (or co-pumped) BDFA system 101 in a communications environment. As shown in FIG. 1B (similar to FIG. 1A), the embodiment of BDFA system 101 includes an amplifier stage 102, shown for illustrative purposes as an O-band amplifier stage operating in the wavelength range of about 1260 nm to about 1360 nm. Those skilled in the art will understand that although O-band is shown as an example, BDFA system 101 and amplifier stage 102 may also be operated in E-band, S-band, or any other operating wavelength band in which a BDFA may be configured to operate.

[0030] The amplifier stage 102 includes a BDF 104. Again, for purposes of illustration, the BDF 104 is shown as a 220 m BDF having a bismuth (Bi)-doped phosphorus silicate glass BDF core. The amplifier stage 102 comprises an output FBG 106 optically coupled (e.g., by fusion splicing or other known optical coupling process) to the BDF 104 at the BDF output, and a WDM 108 having a WDM output optically coupled to the BDF 104 at the BDF input. In the embodiment of FIG. 1B, the input to the WDM 108 (or WDM input) is a pump input 110 to the amplifier stage 102.

[0031] Amplification stage 102 further comprises an input optical isolator 112 that is also optically coupled to the WDM input, and an output optical isolator 116 that is optically coupled to the output FBG 106. For the embodiment of Figure 1B, the input to input optical isolator 112 is also amplifier stage signal input 114, and the output of output optical isolator 116 is amplifier stage output 118.

[0032] In addition to the amplifier stage 102, the BDFA system 101 includes a conversion stage 120 that pumps the amplifier stage 102. The conversion stage 120 includes a pump LD 122, shown for illustrative purposes as a ten-watt (10W) rated multimode (MM) uncooled pump LD for providing pump light with an operating center λ of approximately 915 nm. The conversion stage 120 further includes an HR FBG 124 optically coupled to the pump LD 122. Optically coupled to the HR FBG 124 is a gain-doped optical fiber 126, which is shown in FIG. 1B as 30 m of double-clad YDF. The output of the gain-doped fiber 126 is optically coupled to an OC FBG 128. While the OC FBG 128 is present at the conversion stage output 130, the OC FBG 128 is optically coupled to the input of the WDM 108 located at the amplifier stage input 110.

[0033] Finally, the amplifier stage 102 is pumped by a fiber laser-based conversion stage 120, thereby enabling overpumping of the BDF 104, which is not practically feasible with conventional SMF-QD semiconductor pumps. Furthermore, the power consumption of the disclosed BDFA system 101 is similar to that of a conventionally pumped BDFA operating over the same portion of the O-band at approximately 20°C. At approximately 45°C, the power consumption of the disclosed BDFA system 101 is 1.7 times lower than that of a conventionally pumped BDFA. At approximately 70°C, the power consumption of the disclosed BDFA system 101 is more than 3.5 times lower than that of a conventionally pumped BDFA.

[0034] 1B as a tunable laser source (TLS), and a signal receiver 137, shown as an optical spectrum analyzer (OSA) 137. The signal transmitter 135 is optically coupled to the amplifier stage signal input 114 via a VOA 140, and the signal receiver 137 is optically coupled to the amplifier stage output 118 (at the output optical isolator 116).

[0035] Another embodiment is the backward-pumped (or counter-pumped) BDFA configuration shown in FIG. 2. Specifically, FIG. 2 is a block diagram illustrating one embodiment of a backward-pumped (or counter-pumped) BDFA system 200. Similar to the forward-pumped BDFA system of FIG. 1B, the backward-pumped BDFA system 200 of FIG. 2 includes an amplification stage 202 and a conversion stage 220. For illustrative purposes, both the amplification stage 202 and the conversion stage 220 are shown for O-band operation. However, it should be understood that the operating wavelength ranges and system parameters can be modified to accommodate amplification in other bands, such as, for example, the S-band, the E-band, or other wavelength ranges conducive to BDF amplification.

[0036] In the BDFA system 200 of FIG. 2, the amplifier stage 202 includes a BDF 204 having characteristics (e.g., BDF core diameter, BDF cladding diameter, phosphosilicate glass prepared by MCVD, cutoff wavelength, NA, etc.) comparable to those of the BDF 104 of FIGS. 1A and 1B, with the main difference being that a 180-m-long BDF 204 is used in the backward-pumped BDFA system 200. Subsequently, the amplifier stage 202 further includes an input FBG 206 optically coupled to the BDF input and a WDM 208 optically coupled to the BDF output. Because the amplifier stage 202 is backward-pumped, the output of the WDM is coextensive with the amplifier stage pump input 210. In some embodiments, the input FBG 206 has a reflectivity of approximately 99.9% to prevent pump leakage, improve amplifier gain, and improve NF.

[0037] 2, one embodiment of amplifier stage 202 of Figure 2 further comprises an input optical isolator 212 and an output optical isolator 216. Input optical isolator 212 is at amplifier stage signal input 214, and output optical isolator 216 is located at amplifier stage output 218.

[0038] The conversion stage 220 pumps the amplifier stage 202 and includes a pump LD 222, which (similar to the pump LD 122 in FIGS. 1A and 1B ) is shown for convenience of illustration as a 10 W rated MM uncooled pump LD for providing pump light with an operating center λ of approximately 915 nm. The conversion stage 220 further includes an HR FBG 224 optically coupled to the pump LD 222. In some embodiments, the HR FBG 224 has a reflectivity of approximately 99%. Optically coupled to the HR FBG 224 is a gain-doped optical fiber 226, which is shown in FIG. 2 as a 30 m double-clad YDF with a core diameter of approximately 6 μm and a cladding diameter of approximately 125 μm. The output of the gain-doped fiber 226 is optically coupled to an OC FBG 228. In some embodiments, the HR FBG 224 is written with approximately 99.9% reflectivity and a 3 dB bandwidth of 1 nm, and the OC FBG 228 is written with approximately 75% reflectivity and a 3 dB bandwidth of 1 nm, thereby enabling lasing in the gain-doped fiber 226. To the extent that the OC FBG 228 is present at the conversion stage output 230, the OC FBG 228 is optically coupled to the output of the WDM 208 located at the amplifier stage pump input 210.

[0039] The backward-pumped BDFA system 202 further comprises a signal transmitter 234, shown in Figure 2 as a tunable laser source (TLS), and a signal receiver 236, shown in Figure 2 as an optical spectrum analyzer (OSA) 236. The signal transmitter 234 is optically coupled to the amplifier stage signal input 214 via a VOA 240, and the signal receiver 236 is optically coupled to the amplifier stage output 218 (at the output optical isolator 216). While the TLS, OSA, and VOA 240 are described in detail with reference to Figures 1A and 1B, only a brief discussion of the TLS, OSA, and VOA will be provided with reference to Figure 2.

[0040] For several embodiments demonstrating the suitability of the backward-pumped BDFA system 200 for near-real-time network data transmission, similar operating parameters were used as discussed with reference to FIG. 1A. Specifically, the backward-pumped BDFA system 200 used a 400 Gb / s LR4 QSFP-DD pluggable module with a nominal distance of 10 km. The module was configured with 16 x 25 Gb / s 2 31 The QSFP-DD signal was inserted into an ONT, which generated a 100-Mbps PRBS data lane. Inside the module, the lanes were converted to 4 x 50 Gbaud / s PAM signals and encoded onto four CWDM channels (approximately 1272 nm, 1292 nm, 1310 nm, and 1330 nm) using an external modulator. This baud rate and modulation format were chosen for their high sensitivity to noise. At the receiving end of the QSFP-DD, the WDM channels were separated by an optical filter with a 3 dB bandwidth of 38 nm and converted back to an electrical OOK signal.

[0041] The backward-pumped BDFA system 202 exhibited a maximum output power of 21 dBm at about 1300 nm for an input power of 0 dBm, with a corresponding NF of 5.1 dB. Specific measurement data collected for the backward-pumped BDFA system 202 is shown in plot 600 of FIG. 6. Specifically, plot 600 shows both the gain and NF at operating temperatures of about 20° C. and about 70° C. for input powers of −10 dBm and 0 dBm.

[0042] Any process description or block in the flowchart should be understood as representing a module, segment, or portion of code that includes one or more executable instructions for implementing a particular logical function or step in the process; alternative implementations in which functions may be performed in a different order than that shown or described, including substantially simultaneously or in reverse order, are included within the scope of preferred embodiments of the present disclosure, depending on the functions involved, as will be understood by those skilled in the art of the present disclosure.

[0043] While exemplary embodiments have been shown and described, it will be apparent to those skilled in the art that many changes, modifications, or variations may be made to the present disclosure as described. Accordingly, all such changes, modifications, and variations should be considered within the scope of the present disclosure.

Claims

1. an amplification stage; a conversion stage; The amplification stage comprises: a fiber Bragg grating (FBG) comprising a reflectivity of approximately ninety-nine percent (approximately 99%); a wavelength division multiplexer (WDM); a bismuth-doped optical fiber (BDF) optically coupled between the FBG and the WDM; The BDF is a BDF core having a diameter of about 8 micrometers (about 8 μm); a BDF cladding having a cladding diameter of about 125 μm; a numerical aperture (NA) of about 0.13 (approximately 0.13); a cutoff wavelength (λ) of about 1180 nanometers (about 1180 nm); The conversion stage is configured to pump the BDF, the conversion stage comprising: a 10 watt (10W) rated multimode (MM) uncooled pump laser diode (LD) for providing pump light at an operating center λ of about 915 nm, the 10W rated MM uncooled pump LD operating at a threshold current of about 510 milliamps (about 510 mA) to about 630 mA, operating in a preferred operating temperature range of about 20°C to about 70°C, and including an optical output power to current input slope (LI slope) efficiency of about 0.88 to about 0.82; a high-reflection (HR) FBG optically coupled to the 10 W rated MM uncooled pump LD, the HR FBG having a reflectivity of about 99%; a double-clad (DC) ytterbium (Yb)-doped optical fiber (YDF) configured to convert the pump light from a center wavelength of about 915 nm to about 1150 nm and pump the BDF; The YDF is A YDF core having a diameter of approximately 6 μm; a YDF inner cladding having a diameter of about 125 μm; a YDF input optically coupled to the HR FBG; YDF output, The conversion stage comprises: an output coupler (OC) FBG optically coupled to the YDF output, the OC FBG further optically coupled to the WDM, the OC FBG comprising a reflectivity of about 75%; 1. A bismuth-doped fiber amplifier (BDFA) system comprising:

2. a signal transmitter optically coupled to the input of the amplifier stage; 10. The system of claim 1, further comprising a signal receiver optically coupled to an output of the amplifier stage.

3. 3. The system of claim 2, further comprising a single mode transmission fiber (SMF) optically coupled between the signal transmitter and an input to the amplifier stage.

4. The system of claim 1 , wherein the BDF is configured for overpumping.

5. an amplification stage; a conversion stage providing pump power for overpumping the amplification stage; The amplification stage comprises: A bismuth-doped optical fiber (BDF), A BDF core; BDF cladding, BDF input, BDF output, a bismuth-doped optical fiber comprising: A wavelength division multiplexer (WDM), comprising: a WDM output optically coupled to the BDF input; a WDM input at the amplifier stage pump input; and a wavelength division multiplexer (WDM) including: The conversion stage comprises: a pump laser diode (LD); a high-reflection (HR) fiber Bragg grating (FBG) optically coupled to the pump LD; A gain-doped optical fiber, a gain-doped optical fiber input optically coupled to the HR FBG; a gain-doped optical fiber output; a gain-doped optical fiber including: an output coupler (OC) FBG at the output of the conversion stage, the OC FBG optically coupled between the gain-doped optical fiber output and the WDM input; 1. A forward-pumped bismuth-doped fiber amplifier (BDFA) system comprising:

6. The system of claim 5 , wherein the BDF is configured for overpumping.

7. 6. The system of claim 5, further comprising an output FBG optically coupled to the BDF output.

8. A signal transmitter; 6. The system of claim 5, further comprising a single mode transmission fiber (SMF) optically coupled between the signal transmitter and an input to the amplifier stage.

9. 10. The system of claim 8, further comprising a signal receiver optically coupled to an output of the amplifier stage.

10. 10. The system of claim 9, wherein the signal receiver comprises a quad small form factor pluggable (SMFP) double density (DD) receiver.

11. 10. The system of claim 9, further comprising a bandpass filter (BPF) optically coupled between the output of the amplifier stage and the signal receiver.

12. The system of claim 11 , wherein the signal receiver comprises a quad small form factor pluggable (SMFP) double density (DD) receiver.

13. an amplification stage; a conversion stage providing pump power for overpumping the amplification stage; The amplification stage comprises: A bismuth-doped optical fiber (BDF), A BDF core; BDF cladding, BDF input, BDF output, a bismuth-doped optical fiber comprising: A wavelength division multiplexer (WDM), comprising: a WDM input optically coupled to the BDF output; WDM output at the amplifier stage pump input and and a wavelength division multiplexer including: The conversion stage comprises: a pump laser diode (LD); a high-reflection (HR) fiber Bragg grating (FBG) optically coupled to the pump LD; A gain-doped optical fiber, a gain-doped optical fiber input optically coupled to the HR FBG; a gain-doped optical fiber output; a gain-doped optical fiber including: an output coupler (OC) FBG optically coupled between the gain-doped optical fiber output and the WDM output; 1. A backward-pumped bismuth-doped fiber amplifier (BDFA) system comprising:

14. The system of claim 13 , wherein the BDF is configured for overpumping.

15. 15. The system of claim 14, further comprising an input FBG optically coupled to an output of the amplifier stage.

16. 14. The system of claim 13, further comprising a signal transmitter optically coupled to an input of the amplifier stage.

17. A signal transmitter; 14. The system of claim 13, further comprising a single mode transmission fiber (SMF) optically coupled between the signal transmitter and an input to the amplifier stage.

Images